1. Field of the Invention
The present invention relates to a motor driving technique using pulse width modulation (PWM), and particularly to a control technique for starting up the motor.
2. Description of the Related Art
In recent years, increase in the operation speed of personal computers and workstations has led to rapid increase in the operation speeds of computation LSIs (Large Scale Integrated Circuit) such as CPUs (Central Processing Unit), DSPs (Digital Signal Processor), etc. Such LSIs have a problem in that an increase in the operation speed, i.e., an increase in clock frequency involves an increase in heat generation. The heat generation of the LSI leads to thermal runaway of the LSI itself, or affects its peripheral circuits, which becomes a problem. Accordingly, such a situation requires a suitable thermal cooling operation for the LSI, as a crucial technique.
Examples of techniques for cooling an LSI includes an air-cooling cooling method employing a cooling fan. In this method, for example, a cooling fan is arranged such that it faces the surface of the LSI, and cool air is blown onto the surface of the LSI using the cooling fan.
In some arrangements in which a motor (which is also referred to as a “fan motor”) which is a component of a cooling fan is driven, the time in which current is applied to the motor (the duty ratio) is controlled by pulse width modulation, thereby controlling the torque of the motor, i.e., the rotational speed.